A virtual circuit is a communications arrangement in which data from a source node may be passed to a destination node over more than one communications network route during a single period of communication, where the switching between routes is hidden from the network customers. Virtual circuit techniques are widely used for packet-switched networks and frame relays. A virtual link, or a permanent virtual circuit, can be established as an option to allocate communication resources for repeated use between two customer nodes on a network. Virtual links are usually very seldom broken or disconnected. By configuring virtual links for customers, a network service provider can ensure that adequate resources are available for customer usage. Virtual links can be configured for a customer based on the customer's security policy, such as a need to communicate repeatedly and confidentially between offices, a district headquarters, and a regional headquarters.
A virtual private network (VPN) is a communications protocol often used by customers to communicate confidentially over a public network. VPN traffic can be carried over a public network infrastructure, such as the Internet, on top of standard protocols, or over a network service provider's private network with a defined service level agreement between the VPN customer and the VPN network service provider. A VPN can send data such as voice, data or video, or a combination of these media, across a secured and encrypted private channel, such as a virtual link, between two nodes. Each node can function as both a customer source node and a customer destination node.
Layer 2 Tunneling Protocol (L2TP) is a communications protocol used to support virtual private networks. L2TP acts like a data link layer protocol for tunneling network traffic between two nodes over an existing network, such as the Internet. Once a tunnel is established for a virtual link, the network traffic between the nodes is bidirectional. To be useful for networking, higher-level protocols are then run through the L2TP tunnel. To facilitate this, an L2TP session, or call, is established within the tunnel for each higher-level protocol such as point to point protocol. The traffic for each session is isolated by L2TP, so it is possible to set up multiple VPNs across a single tunnel or virtual link.